Quartz resonator pressure transducers have been used successfully in the downhole environment of oil and gas wells for several decades and are still the most accurate means of determining bottom-hole pressure. While many measurements of these downhole pressures are made under static or slowly varying pressure and temperature conditions, some significant situations, however, require pressure measurements under transient conditions where either or both of the temperature and pressure are changing. The range of static to dynamic measurement conditions, the economic drive for less expensive devices, and the increasing levels of pressures and temperatures arising as the world oil and gas exploration and production industry drills deeper and deeper, have spurred continuing developments in quartz resonator pressure transducers.
The first commercially successful quartz resonator pressure transducer, as disclosed in U.S. Pat. Nos. 3,561,832 and 3,617,780, the disclosure of each of which is hereby incorporated herein in its entirety by this reference, was introduced by Hewlett Packard (“HP”) in the 1970's. This transducer was of a cylindrical design with the resonator formed in a unitary fashion in a single piece of quartz. End caps of quartz were attached to close the structure. FIG. 1A shows this configuration, which contains resonator 1a unitary (integral) with body 2a, two end caps 3a, and two glass joints 4a. This device was relatively large, approximately 1 inch diameter and 4 inches long. The unitary body and resonator are expensive to manufacture. Also, two major disadvantages were caused by the large size. Large stress distributions occur throughout the structure under transient conditions because the temperature distribution is slow to equilibrate. These stresses cause errors in the pressure measurement. Also, it is not practical to obtain a temperature measurement, necessary for temperature compensation, close to the actual location of the pressure measurement, e.g., the resonator, because of the large transducer size. This lack of proximity results in temperature errors in transient conditions because the temperature at a temperature transducer used to temperature compensation may not be the same as the required temperature located at the resonator itself. Both of these problems restricted the use of this concept to the more benign, nearly static cases.
A somewhat smaller size transducer was introduced in the 1980's by Quartztronics, Inc., of Murray, Utah, and commercialized by Halliburton Company through its Halliburton Services operating unit, now part of Halliburton Energy Services. This device, as described in U.S. Pat. Nos. 4,550,610 and 4,660,420, the disclosure of each of which is hereby incorporated herein in its entirety by this reference, was similar to the unitary HP design, except diametrically opposed flats were added to the cylindrical shape to create a non-uniform stress distribution in the resonator under pressure. FIG. 1B shows this structure, which contains resonator 1b unitary with body 2b, two end caps 3b, two glass joints 4b, and a pair of flats 5b (backside flat not shown in FIG. 1B). The smaller size of the Quartztronics transducer reduced the cost, and the flats increased the pressure sensitivity while reducing the temperature sensitivity. The smaller size also reduced the amount of undesired stress distribution from non-uniform thermal distributions and enabled temperature to be measured closer to the pressure measurement location.
Another quartz resonator transducer design was introduced in the 1990's by Quartzdyne, Inc. of Murray, Utah. This device eliminated the body/resonator unitary structure by simply bonding a convex-convex resonator between two end caps. FIG. 1C shows this configuration, which contains resonator 1c, two endcaps 3c and two glass joints 4c. Besides low cost, the physical size was small enough to move the temperature measurement point to within a few millimeters of the pressure measurement location.
The foregoing three quartz resonator transducers each use a single resonant mode, the slow-shear thickness mode, or C-mode, to determine pressure external to the transducer. A temperature compensation signal is supplied with an independent temperature measurement device located as close as possible to the pressure measurement (resonator) location.
In light of recognition of a need for good pressure measurements in transient conditions, researchers have explored different ways to use a dual-mode transducer, wherein two resonant modes are driven by the driving circuits of the transducer at the same time. In a dual-mode transducer, one resonant mode is mainly dependent on pressure, the other mode is mainly dependent on temperature. This approach would provide a temperature measurement located exactly where the pressure measurement was made, eliminating one important error source. One mode, usually the C-mode, is used to measure the pressure, and a second mode, the fast-shear mode, or B-mode, is used to determine the temperature. With the two unknowns, pressure and temperature, and two simultaneous measurements, one can solve the two equations. However, during a transient condition, the non-uniform stresses in the structure, arising from non-uniform temperature distributions therein, changes the resonant frequencies. If the B-mode is pressure sensitive, this frequency error will cause an error in the temperature calculation which will, in turn, cause an error in the calculation of the pressure. One is forced to perform a series of iterative calculations that may not lead to accurate pressure and temperature answers. The simplicity and accuracy of the pressure calculation in this case is greatly enhanced if the B-mode is not pressure sensitive. This fact has driven research efforts in dual-mode transducers to find B-modes with no pressure sensitivity while still having a C-mode available for the pressure measurement.
It has been recognized that one way to obtain a B-mode that is independent of pressure is to change the crystallographic orientation of the quartz in the device. This approach led to the SBTC orientation, as described in Michel Valdois, Bikash K. Sinha, and Jean Jacques Boy, EXPERIMENTAL VERIFICATION OF STRESS COMPENSATION IN THE SBTC-CUT, IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 36, p. 643, 1989, the disclosure of which is hereby incorporated herein in its entirety by this reference. The shape of the SBTC orientation transducer is identical to that shown in FIG. 1A. Although this concept was successful in obtaining a B-mode with no pressure sensitivity, the C-mode was not usable in a practical oscillator circuit because of high electrical resistance.
Another approach to quartz resonator transducer design is described in U.S. Pat. No. 4,562,375, the disclosure of which is hereby incorporated herein in its entirety by this reference. This structure uses a resonator bonded between two end caps, similar to the structure depicted in FIG. 1C. However, the resonator in this transducer design includes slots to isolate most of the perimeter of the resonator, leaving small bridges to transfer the force from the endcaps along a specific direction such that the B-mode will be pressure insensitive. This structure has never been used commercially. The reason is believed to be that the design cannot withstand the high pressures experienced in a wellbore without failure because of the stress concentrations in the corners of the slots.
To date, the only commercially successful dual-mode quartz pressure transducer is the CQG (Crystal Quartz Gauge), offered by Schlumberger and described in U.S. Pat. Nos. 4,547,691, 5,394,345 and 6,147,437, the disclosure of each of which is hereby incorporated herein in its entirety by this reference. It is a radical departure from the previous structures in that, although the exterior is essentially cylindrical, the resonator is suspended across the inside diameter with the plane of the resonator extending in the axial direction. FIG. 2 shows the CQG structure, which contains resonator 1d unitary with body 2d, two end caps 3d, and two glass joints 4d. The drawings of the patents relating to this structure show it with no flats, as well as with flats 5d. There is an additional small flat 6d, as shown. This small flat 6d is shallow enough that it does not appreciably affect the stress magnitude or distribution in the resonator 1d, and is apparently used for assembly facilitation to crystallographically orient the end caps 3d with the body 2d. Whereas all previous transducer structures previously mentioned herein exhibit a two-dimensional stress in the resonator, the CQG structure has an almost uniaxial stress pattern in the resonator. The orientation of the resonator can be selected so that the B-mode is pressure insensitive.
There have been several attempts to accomplish dual-mode operation in a structure resembling the Quartztronics design employed by Halliburton, where the stress in the resonator is two-dimensional, but not uniform. One approach is described in U.S. Pat. No. 6,455,985, the disclosure of which is hereby incorporated herein in its entirety by this reference. In this design, the unitary body with the resonator is cylindrical. However, the end caps, while being cylindrical on the outside, are stiffened inside along one direction to create a non-uniform stress in the resonator. A second approach is described in U.S. Pat. No. 6,111,340 (the “'340 patent”), the disclosure of which is hereby incorporated herein in its entirety by this reference. In this design, the structure is the same as the shape employed in the Quartztronics/Halliburton transducer, the only difference being that it is a dual-mode device. However, the '340 patent demonstrates that, even with very deep flats that take up two-thirds (⅔) of the wall thickness, it is not possible to render the B-mode completely independent of pressure but suggests that, if the pressure sensitivity of the B-mode can be reduced sufficiently with the use of the flats, a usable device is possible. This invention would appear to be useful only if the flats are very deep. However, the stress concentrations associated with deep flats may lead to cracking or twinning, and are not consistent with an ongoing desire prevalent throughout the industry to extend the upper limits of pressure and temperature measurement.
As disclosed in Schodowski, RESONATOR SELF-TEMPERATURE-SENSING USING A DUAL-HARMONIC-MODE CRYSTAL OSCILLATOR, 43rd Annual Symposium on Frequency Control, 1989, p. 2 and U.S. Pat. No. 4,872,765 to Schodowski as well as in U.S. Pat. No. 4,545,638 to EerNisse and Ward, the disclosure of each of which is hereby incorporated herein in its entirety by this reference, temperature compensation is accomplished by using two harmonically related resonances, typically the C-mode fundamental and 3rd overtone. The temperature is calculated using the formula 3*fCfund−fC3rd. This use of harmonically related vibrational modes must, however, include the fundamental mode to obtain the temperature sensitivity. As the fundamental mode is more spread out than the overtones, a device employing this approach requires a relatively large resonator bore diameter, leaving many unwanted modes not clamped and increasing the chances for an activity dip.
One limitation common to all the quartz resonator pressure transducer concepts is a tendency toward twinning at high applied stress and temperature. Twinning is not reversible and renders the device unusable. In the past few years, the pressures and temperatures encountered in the deeper wells have exceeded the capabilities of the CQG structure, which has stress concentrations in edges and corners. Twinning is less prevalent in designs such as those of FIGS. 1A and 1C with uniform two-dimensional stress in the resonator. Also, because these cylindrical structures minimize the number of corners and edges, cracking and twinning in the rest of the structure is less probable.